1. Field of the Invention
The subject invention relates to an assembly including a power supply for supplying electrical power to an array of light emitting diodes (LEDs).
2. Description of the Prior Art
Light emitting diode (LED) signals are rapidly replacing conventional incandescent lamps in a variety of applications. Many LED signals, such as those for automotive uses, are directly operated from low voltage d.c. power sources. On the other hand, LED signals specifically designed to operate from the a.c. mains are becoming more common. These a.c. line operated devices, such as traffic signals usually include an integral a.c. to d.c. power supply to operate the LEDs. First generation power supplies for LED traffic signals consisted of simple reactive (capacitor) current limited circuits coupled to a full wave rectifier, ballast resistors and a network of series-parallel connected LEDs. The poor power factor and distortion performance of such simple power supplies, coupled with minimal line or load regulation has made their use unlikely for all but the least sophisticated, non safety critical applications. Second generation a.c. power supplies for LED signals usually employed linear current regulation, to accommodate some variance in power line supply voltage. The linear control element, usually a transistor and a power resistor was naturally dissipative and added undesirable heat to the LED signal assembly. Such self generated heat, when added to normal environmental heat, proved to be deleterious to the LED signals, which degraded rapidly in service.
Recent regulatory initiatives designed to assure the safety and quality of LED signals for traffic applications [Institute of Transportation Engineers, Interim LED Purchase Specification, July, 1998] have established minimum performance criteria for LED based signals. Among the specified performance parameters is a requirement for the LED signal to maintain a minimum luminous intensity over a relatively wide range of a.c. line voltage (85 to 135 Volts). The specified operating temperature range of -40.degree. C. (-40.degree. F.) to 74.degree. C. (167.degree. F.) is related to signal visibility issues and driver safety, and is necessary because most common (red) LEDs exhibit a diminution in luminous output of approximately -1% per .degree. C. increase in temperature. That is, using 25.degree. C. as reference point, an uncontrolled LED signal might lose about 50% of its initial brightness when operated at 74.degree. C. Such elevated temperatures have been shown to be rather common in traffic signal enclosures that are placed in service and are exposed to direct sunlight.
Third generation LED traffic signals are now available with efficient, switch mode power supplies that also provide power factor correction, and the necessary line regulation. When equipped with luminous output maintenance control circuitry as shown in U.S. Pat. No. 5,661,645, the power supply and control circuitry acting together can meet the proposed performance specifications for LED traffic signals.
Typically, the off line, switch mode power supplies used in existing traffic signals deliver between 100 volts and 300 volts of regulated d.c. to the LED array. The large number of LEDs necessary to meet the specified luminous output, suggests the use of long series strings of parallel connected LEDs. That is, the nominal 1.7 volt forward voltage drop across each LED (at 20 mA) requires some fifty eight devices to be connected in series. To prevent one local device failure from extinguishing the entire string, two or more LEDs are commonly connected in parallel, in a rudimentary current sharing arrangement.
For traffic signal applications, using nominally 1.2 Cd output LEDs (operated at 20 mA) a total of 180 LEDs were typically needed to fulfill the luminous requirements of an eight inch (200 mm) red LED traffic signal, while three hundred sixty devices would satisfy the requirements for twelve inch (300 mm) red signals. Of course, many other parameters influence the number of LEDs chosen for a particular application. Operating temperature, thermal management, permissible operating current and projected safe life are among some of the design variables.
Recently, very high luminous output LEDs have become commercially available because of advances in LED fabrication technology. Typically, these larger, copper heat sinked devices can provide up to ten times the light output of older, steel lead frame LEDs, albeit at four times the operating current. Reducing the number of LEDs in a signal assembly virtually ten fold, has dramatic implications for manufacture, reliability and naturally cost.
A simple step-down transformer, full wave rectified power supply could be designed to deliver the requisite voltage and current at very low cost. It would not provide the necessary line regulation nor would it compensate for the diminution in light output from the LEDs as they heated up. Of course, a fixed or programmable linear regulator could be used to provide the required regulation, but at a significant penalty in terms of power dissipation and temperature rise.
By means of example, assuming that proper operation at a reduced line voltage of 85 Volts is required (120 V. being the nominal design to voltage), then an approximately 30% increase in secondary transformer voltage would be required to maintain the 11.8 Volt d.c. supply. Taking into account the loss of luminous intensity with temperature approximately 50% increase in operating current may be required at 74.degree. C. compared to the requisite current at 25.degree. C. That is, to properly compensate for both specified line voltage variation and the added current needed to maintain luminous output at high temperature, the power supply would have to exhibit a nearly 80% adjustment range. Building in such voltage overhead with linear regulation is terribly inefficient. Minimally, a secondary d.c. voltage of 1.80.times.11.8 Volts or 21.2 Volts would be necessary. At nominal line voltage (120 V.A.C.) the difference between 21.2 Volts and the 11.8 Volt operating voltage (at 25.degree. C.) would result in a dissipation of 3.3 Watts, which while not significant in and of itself, is a rather large percentage (87%) of the LED load power of 3.8 watts. Not accounting for transformer efficiency, the net power supply efficiency would be under 60%.
A low voltage, switch mode regulator could be used instead of the linear regulator postulated above, but the added cost, complexity and reduced reliability of this approach is not always commercially attractive.
Adjustable transformers have been used since the advent of alternating current power systems, since such devices are extraordinarily efficient. Mechanical turns changing transformers and adjustable tap switching transformers are used today in high power electrical distribution systems to compensate for line voltage variations, U.S. Pat. Nos. 5,408,171; 5,006,784 and 3,944,913 being examples of this art. The present invention addresses the problem of an adjustable, efficient, line transformer powered LED signal with a novel approach. U.S. Pat. No. 4,454,466 to Ritter discloses a tap switching transformer but does not suggest the combination with light emitting diodes. Other U.S. Pat. No. 4,717,889 to Engelmann, U.S. Pat. No. 4,816,738 to Nicolas, U.S. Pat. No. 4,896,092 to Flynn and U.S. Pat. No. 5,633,580 to Trainor et al also suggest tap switching transformers but not in combination with light emitting diodes to maintain the luminosity of the LEDs.